Physical vapor deposition (PVD) is directed to a variety of vacuum deposition methods which can be used to produce thin films and coatings. Such processes typically require the use of hostile gases at low pressures within a vacuum chamber. These processes include, for example, plasma deposition, plasma etching, low pressure chemical vapor deposition and ion implantation.
Historically, many PVD processes utilized oil diffusion high vacuum pumps. However, many industries have moved away from the use of oil diffusion pumps as part of PVD process equipment due to contamination. That is, the operating principle of oil diffusion pumps dictates that the working oil of the pump be directly exposed to the chamber that is evacuated. Oil molecules thus migrate into the process chamber, intermingle with the gases and contaminate the process. For many PVD applications where contamination is a concern (e.g., semi-conductor manufacturing), turbomolecular pumps have become the industry standard, as such pumps typically do not contaminate the local vacuum environment.
Turbomolecular pumps are basically high-speed turbines that operate on kinetic gas principles. That is, turbomolecular pumps work on the principle that gas molecules can be given momentum in a desired direction by repeated collision with a moving solid surface. In a turbomolecular pump, rapidly spinning rotor blades ‘hit’ gas molecules from the inlet of the pump towards the exhaust in order to create or maintain a vacuum. Most turbomolecular pumps employ multiple stages, each consisting of a set or stack of rotating rotor blades/vanes and stationary stator blades/vanes. Typically, the rotor of a turbomolecular pump rotates on sealed and/or magnetic levitation bearings, which results in little or no contamination. In any arrangement, gas molecules captured by the upper stages of the pump are pushed into the lower stages and successively compressed. As the gas molecules enter through the inlet, the rotor, which has a number of angled vanes, hits the molecules. Thus the mechanical energy of the vanes is transferred to the gas molecules. With this newly acquired momentum, the gas molecules enter into the gas transfer areas in the stator vanes. This leads them to the next stage where they again collide with a rotating rotor vane surface, and this process is continued, finally leading the molecules outwards through the exhaust.
To achieve low vacuum levels, turbomolecular pumps run at high rotational speeds. In order to obtain extremely low pressures down to, for example, 1 micropascal, rotation rates of 20,000 to 90,000 revolutions per minute are often necessary. Such high rotation rates stress the rotor bearings of the pump requiring periodic maintenance and/or replacement of the bearings. Bearing can be further stressed in PVD processes where a deposition process generates metal or chemical vapors that can deposit/condense on the turbo pump rotor blades. Such deposition/condensation can create an unbalanced condition for the turbo pump rotor. Such an unbalanced condition can significantly shorten the life of the bearings thereby requiring early bearing replacement and/or rebalancing of the rotor.